These days, an optical system such as a laser printer, an optical head etc. in which a plurality of light beams radiated from a plurality of light emitting sources are focused on a recording medium by a focusing lens has been studied for the purpose of a high speed and functionable performance based on a parallel light beam operation.
This type of an optical head in which two light beams are focused on a recording medium comprises a semiconductor laser array for emitting two light beams, a collimating lens for collimating the two light beams, a beam splitter for transmitting the two light beams therethrough and reflecting two light beams reflected from a recording medium, a focusing lens for focusing the two light beams transmitted through the beam splitter on the recording medium, and an optical detecting system in which the two light beams reflected from the beam splitter are received to read information from the recording medium and to detect focusing and tracking errors.
In operation, the two light beams are radiated from the semiconductor laser array, and then collimated by the collimating lens. The two light beams thus collimated are transmitted through the beam splitter, and then focused on the recording medium by the focusing lens. The two light beams are reflected from the recording medium, and then collimated by the focusing lens. The two light beams propagated in the reverse direction are reflected in the direction of the optical detecting system by the beam splitter, so that information is read from the recording medium in the optical detecting system, and focusing and tracking errors are detected therein. The focusing lens is controlled to be corrected in its position based on the focusing and tracking errors by an actuator.
There is another optical head which comprises first and second semiconductor lasers in place of the semiconductor laser array, and further comprises a collimating lens for collimating light beam radiated from the second semiconductor laser, a wedge prism positioned on a light path of the light beam from the second semiconductor laser and for rotating a line connecting two beam spots defined on the recording medium by focusing light beams from the first and second semiconductor lasers thereon, and an optical combiner for combining the two light beams from the first and second semiconductor lasers.
In operation, the light beam which is radiated from the first semiconductor laser and then collimated by the collimating lens, and the light beam which is radiated from the second semiconductor laser and then collimated by the added collimating lens are combined by the optical combiner, so that the two light beams thus combined are transmitted through the beam splitter to perform the same operation as that explained in the former optical head.
In the former and latter optical heads, the two light beams which are radiated from the semiconductor laser array and the first and second semiconductor lasers are utilized. In accordance with the adoption of, for instance, four light emitting sources, four light beams are utilized, so that four beam spots are defined on four corresponding tracks of the recording medium. Even if the four beam spots are exactly positioned on the corresponding tracks, some of the spots are deviated off the tracks due to the mechanical precision of the optical head when it is moved to access the recording medium. Such a deviation of beam spots are corrected in accordance with the rotation of a whole optical head in case of the former optical head, and an angular motion of the wedge prism in case of the latter optical head, so that an angle of the line connecting the two beam spots is controlled relative to a track of the recording medium to result in the exact position of the beam spots on corresponding tracks. The wedge prism system as explained above is described on pages 121 to 126 of the report in "Optical memory symposium 86" held on Dec. 17, 1986 in Tokyo.
According to the former optical head, however, there are disadvantages that a high speed control of moving a whole optical head is not expected and a mechanism thereof becomes large because the whole optical head is relatively heavy and requires a predetermined size.
Further, according to the latter optical head, there are disadvantages that a construction becomes complicated because the number of wedge prisms is increased dependent on that of light beams, and that semiconductor laser array can not be utilized because light beam controlled by a wedge prism must be on a light path independently of other light beams.